This invention relates to a control system for controlling an internal combustion engine, including an air/fuel ratio control system, and more particularly to a device provided in such system for detecting abnormalities in the engine speed detecting system.
It is generally known to detect the operating condition of an internal combustion engine, such as engine speed, load on the engine, acceleration and deceleration, on the basis of engine rpm and absolute pressure in the intake pipe of the engine, to effect control of the air/fuel ratio of an air/fuel mixture being supplied to the engine, ignition timing, engine exhaust emissions, etc.
As one of the above control systems, there has been proposed by the assignee of the present application an air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, which comprises an O.sub.2 sensor for detecting the concentration of oxygen present in exhaust gases emitted from the engine, a carburetor for producing the mixture being supplied to the engine, and means operatively connecting the O.sub.2 sensor with the carburetor in a manner effecting feedback control operation to control the air/fuel ratio of the mixture to a predetermined value, the connecting means comprising an electrical circuit, an air/fuel ratio control valve and a pulse motor for driving the air/fuel ratio control valve in response to an output signal produced by the O.sub.2 sensor.
The above proposed control system is provided with an engine speed signal detecting system comprising a pressure sensor for detecting pressure in the intake pipe of the engine, and an rpm sensor for detecting the engine rpm.
In the event that these sensors produce abnormal outputs owing to failure or the like, it is impossible to properly effect control of the engine on the basis of the outputs of these sensors. Particularly in the above air/fuel ratio control system proposed by the present assignee, the engine is controlled in different manners depending upon the rotational speed of the engine and load on the engine. For instance, the control of the engine includes open loop control which comprises controlling the air/fuel ratio of the mixture to respective predetermined values suitable for various engine operating conditions such as wide-open-throttle, engine idle and engine deceleration when these operating conditions are detected on the basis of the pressure in the intake pipe or the engine rpm, and closed loop control which comprises controlling the mixture air/fuel ratio to a proper predetermined value in immediate response to changes in the output of the O.sub.2 sensor when engine partial load is detected on the basis of the above factors. It goes without saying that in such control system, accurate air/fuel control cannot be carried out in the event of a failure in the pressure sensor or engine speed sensor of the engine speed signal detecting system.